The invention may be used in information systems using a standard TV set.
This invention relates to the technical field of graphic terminals. More particularly, the invention relates to a processor which, connected to a control unit, enables a graphic image to be displayed on CRT a screen.
Information systems which enable a graphic image composed of geometric figures, alphanumeric characters and various symbols to be displayed on a CRT screen are known in the art as graphic terminals. A graphic terminal comprises in particular a display console generally equipped with a cathode ray tube. Display consoles are divided into two classes according to the method by which the cathode screen is scanned: one of these classes includes consoles equipped with a cathode ray storage tube on the screen of which the data of the image are directly recorded by a so-called "random" scan, whilst the other class includes consoles equipped with a low-persistence cathode ray tube, in whose case the data of the image are stored in a modular memory unit which is read repetitively in cycles at a high rate by a "raster" scan in order to avoid flickering of the image displayed on the cathode ray screen. The present invention relates more particularly, but not exclusively, to this second class of graphic image display consoles known in the art. Various systems designs have already been proposed in the literature; cf. in particular the work of P. MORVAN et al "Images et Ordinateurs", published by Larousse, Paris, 1976.
In addition to the graphic TV console and the memory unit, also known as the "image data storage and refreshment memory", a graphic terminal comprises a control unit for reading the content of the image memory and for synchronizing the television scan of the console, a graphic drawing unit for producing the data of the image and recording them in the memory unit, dialogue tools, such as a keyboard, a photostylus or light pen, a control randle, a rolling ball, a graphic tablet, etc.
For numerous reasons, for example for the local processing of data, for mail order work, for access to data banks, there is a need for interactive graphic consoles selling at a relatively low price. This is because, on the one hand, numerous potential users of information systems already have a standard television receiver or TV set; on the other hand, the unit cost of the memory modules required for forming a modular memory unit is relatively low and, by virtue of their repetitive character, their high unit capacity (number of memory bits per module) and the relatively small number of "input/output" plugs required, these memory modules or packages are readily assembled and interconnected.
So far as the control unit and the graphic unit are concerned, the situation is far different. The construction of these units requires the assembly of a very large number of fairly diverse MSI (medium scale integrated) or SSI (small scale integrated) circuits. The extremely diverse character of the necessary packages and the difficulties involved in assembling and interconnecting them result in a high production cost of these units which virtually prevents graphic consoles from being widely used by the public.
Ideally, it would be necessary to construct the assembly formed by the control unit and the graphic unit in the form of a single package in which the corresponding logic circuits would be integrated on a single microchip of a semi-conductor substrate so as to produce a processor which would be connected between the memory unit and a control unit (microprocessor or computer).
The construction of a processor for a graphic TV console on a single microchip involves several problems. The main problem lies in the need to produce a sufficiently versatile processor, i.e. a processor which affords wide possibilities enabling the various fields of application envisaged to be covered. Another problem is to avoid as far as possible any circuit of the analog type and the use of passive components, such as resistors and capacitors. The technological* to be solved relate in particular to the maximum operating frequency, the number of input/output pins of the package, the number of interconnections, the surface area of the microchip, etc. FNT *problems